1. Field of the Invention
This invention relates to the treatment of industrial waste, especially sludge generated by electroplating operations. More particularly, this invention consists of a process and apparatus designed to deal with that form of electroplating waste designated by the U.S. Environmental Protection Agency (EPA) as F006 sludge. The objective achieved by the invention is the stabilization of F006 sludge against leaching--as measured by the Toxic Characteristic Leaching Procedure (TCLP) [40 CFR .sctn. 1.268, Appendix 1 (7-1-88)]--to the degree that the treated material meets EPA leach-resistant requirements for landfill disposal pursuant to the Hazardous and Solid Waste Amendments of 1984 (HSWA) [98 Stat. 3221] and to the Resource Conservation and Recovery Act of 1976 (RCRA) [U.S. Code 1982 Title 42 .sctn. 6901 et seq. Oct. 21, 1976, P.L. 94-580, 90 Stat. 2795].
Industrial waste disposal is becoming ever more tightly regulated, especially with respect land disposal in sanitary land fills. The EPA is required to classify hazardous wastes and to prohibit their land disposal unless certain very stringent conditions are met. For the purposes of the present invention, the wastes generated by industrial electroplating are of particular interest. Such wastes have been divided into different categories, depending on the identity of their major constituents and the specific electroplating process producing them. These categories are denominated as F006, F007, F008, . . . F0mn. [See, for example, EPA/530-SW-88-0009-I, Best Demonstrated Available Technology (BDAT) Background Document for F006, Volume 13 (Proposed), May 1988. ]Although the present invention is directed toward F006 wastes, it can be modified to deal with several of the other F0mn categories as well. F006 sludge is broadly defined in 40 CFR .sctn. 1.268.10 as:
Waste-water treatment sludges from electroplating operations except from the following processes: (1) Sulfuric acid anodizing of aluminum; (2) tin plating on carbon steel; (3) zinc plating (segregated basis) on carbon steel; (4) aluminum or zinc-aluminum plating on carbon steel; (5) cleaning/stripping associated with tin, zinc and aluminum plating on carbon steel; and (6) chemical etching and milling of aluminum.
Electroplating is key to a wide range of industry because it enables one to: (1) provide corrosion protection for a multitude of items; (2) control the surface resistivity of electronic devices; (3) apply a decorative or functional coating to a myriad of items. Since the electroplating industry is a sizable part of the industrial economy and electroplating by its nature creates a high volume of hazardous waste by-products, anything which limits the freedom of the industry to dispose of such by-products has a very high economic impact. Viewing the problem from a different perspective, one notes the extreme importance to society's general well-being that hazardous wastes be disposed of in a manner which minimizes the air and water release of the toxic, mutagenic, teratogenic, and carcinogenic components of that waste. The U.S. Congress through the EPA has ruled that such waste, before it can be deposited in landfills where it eventually will be exposed to leaching agents (primarily water run-off), must be able to pass stringent tests of stability with respect to potential leaching of any "scheduled" compounds. These tests are codified by the EPA in terms of TCLP toxicity levels which the waste must not exceed if it is to be directly deposited into a sanitary land fill--the only practical disposal mode in view of the total annual tonnage involved.
Specifically, the invention calls for thoroughly mixing extraordinarily small quantities of certain metal salts, in particular metal soaps--salts of the fatty acids such as stearic acid, oleic acid, and palmitic acid--with the sludge, extruding the mixture, and then transferring energy to it by induction heating at relatively low temperatures. The water which is forced to the surface of the extruded sludge mixture--as the result of the heating and the formation of hydrophobic bonds--is removed in part by evacuating the region around the product. It is also removed in part by direct mechanical methods, thus reducing the total heat which must be introduced. It is apparently the removal of free and loosely-bound water and the formation of micro-matrices within the extruded sludge that effectively binds the waste's toxic components to the degree that the end product passes the TCLP tests.
2. Description of Prior Art
Even though it is only relatively recently that the F006 disposal situation has become extremely acute, the problem of what to do with heavy-metal-contaminated effluent streams has confronted the metal-plating and electronics industries for years. One early approach to the problem was simply to dry the sludge in large conventional ovens and then to place it in landfills. This action implied a certain obliviousness to the leaching dangers, since conventional drying does not detoxify electroplating sludges to the point where they can be safely deposited in landfills. The inadequacy of this technique is clear in the light of the EPA standards referred to above. Sludges treated simply by conventional heating are not able to comply with the TCLP-based criteria set out by the EPA.
Recognizing the inadequacy of conventional drying, the industry turned to techniques involving the precipitation of heavy metal compounds from the raw sludge so as to produce cleaner material for disposal. Of course, one of the by-products of such precipitation is itself hazardous sludge, sludge which though smaller in volume than the original material, contains a higher concentration of hazardous compounds and hence has a higher specific toxicity. Thus, the precipitation approach simply shifted to a new arena the waste treatment problem presented by the sludge.
It was in that context that the EPA published EPA/530-SW-88-0009-I, Best Demonstrated Available Technology (BDAT) Background Document for F006, which summarized current F006 treatment methods. The methods which that document sets out for treating electroplating waste can be categorized as: stabilization, vitrification, and high-temperature metal recovery. Of these, the only realistic methods at present involve stabilization.
Stabilization methods work by locking the sludge's hazardous materials in place rather than removing or chemical modifying them. Prior stabilization methods have required that one add large quantities of a stabilizing compound such as Portland cement to the sludge, and then cure the mixture. Leaching of metals and other toxic substances from the resulting waste is then impeded by the entrapment of those substances within the solid matrices established throughout the sludge by the stabilizing agent.
There are four serious drawbacks associated with traditional stabilization. First, it requires de-watering of the sludge as a prerequisite. The conventional methods of dewatering sludge include pressing, centrifuging, and conventional heat drying: all are time-consuming and expensive. Secondly, the addition of the stabilizing agent increases the weight and volume of sludge by a great deal, up to 150%. The increase in the amount of waste to be deposited is a serious problem, both in terms of shipping expense and allocation of scarce landfill space. Thirdly, traditional stabilizing methods require a long curing period. In addition to the time required to treat each load of sludge, vast amounts of energy are used by the drying ovens. Lastly, traditional stabilization methods result in a waste product with little physical integrity, a situation which leads to crumbling and the production of large quantities of toxic dust and larger fragments. Not only are the toxic dust and fragments hazardous to personnel transporting and disposing of the waste, but the fragmentation increases the surface area exposed to leaching agents after disposal into the landfill and hence increases the likelihood of subsequent leaching.
An example of a stabilizing process requiring both the pre-treatment of the sludge and the addition of significant amounts of a thermoplastic stabilizing agent is described in U.S. Pat. No. 4,242,220 issued to Sato in 1980. Sato teaches a method of treating waste sludge requiring the following sequence of steps: (1) pre-treating the waste sludge until its water content is not greater than 13%, (2) using microwave radiation to weaken the coalescence between the sludge particles to the point where a powder results, (3) mixing a thermoplastic resin with that powder, (4) using microwave radiation to melt the resin so as to trap the sludge particles in an insoluble capsule, and (5) cooling and molding the mixture into a solid mass. In addition to being limited to sludge with a low water content and the fact that significant volume is added to the waste product, the Sato process requires two drying steps. (It is true that Sato utilizes microwave rather then conventional heating, a fact that within the context of the process provides a reduction in the total electrical energy required. It appears that the use of the heat is to effect physical rather than chemical change in the mixture, in distinction to the use of microwaves in the present applicant's invention.)
An example of a stabilizing process which requires the addition of large quantities of a glass-like stabilizing agent and which ultimately results in a brittle waste product is described in U.S. Pat. No. 4,221,680 issued to Hardwick in 1980. Hardwick teaches a radioactive waste sludge treatment requiring the following sequence of steps: (1) injecting a slurry containing radioactive wastes into open glass slugs, (2) placing the filled slugs in a microwave oven to dry the slurry while venting gases out of the oven and (3) fusing the dried slurry within the glass slug to produces a glass-like solid material. Although the Hardwick process appears to be able to handle sludge with a high water content, the dried slurry is not disposable until it has been fused with the glass slug. Not only does the glass add substantial volume to the waste product, but it has the additional disadvantage of providing only a brittle shell between the environment and the toxic slurry.
Apparatus to extrude sludge onto a belt and through a dryer system are generally recognized art. U.S. Pat. No. 4,043,047 issued to Galliker in 1977 discloses apparatus for reducing watery sludge to a friable mud. This apparatus utilizes a conventional drying process which would not be suitable for F006 sludge. It encompasses a piston pump extruder and an electrolytic heat treatment unit connected by a conveyor belt. It appears that the only way to adapt the Galliker apparatus to handle waste with different characteristics is by adjusting the speed of the pump so as to vary the quantity of materials per unit of time passing through the drying chambers.
What is needed is a waste treatment process that can treat toxic sludge possessing a high and variable water content so as produce a readily disposable, cohesive solid waste product without significantly increasing the total weight or volume to be disposed of. Furthermore, waste treatment apparatus is needed which is easily adapted to the landfill preparation of electroplating wastes possessing a wide variety of physical and chemical characteristics.